It is desirable to have platforms at high altitude to provide communication relay and broadcasting, imaging of the earth's surface, atmosphere and weather monitoring, and numerous other applications. At present, there are few methods available for providing a persistent presence at high altitude. For operation between 55 km and 200 km, there are essentially a limited number of approaches. Aircraft, balloons, airships, and satellites have been tried; however, many existing approaches are considered to be inefficient.
Aircraft such as the U-2, the SR-71, and some unmanned aerial vehicles (UAV) are able to operate up to 40 km in altitude and persist for up to 24 hours. Refueling and alternate power sources may allow persistence indefinitely; however, aircraft must remain in continuous motion, and wing lift is limited to about 40 km in altitude unless operating at extremely high speeds. Rockets may travel higher, but only for a few minutes.
Balloons can operate for long periods of time, but only up to 55 km. Balloons are also subject to wind and have difficulty maintaining position. Prior to 2002, the altitude record for unmanned balloons was 51.8 km (170,000 ft) set by a balloon with a volume of 1.35 million cubic meters launched in October 1972. On May 23, 2002, a Japanese attempt (BU60-1) reached a new record with a 60,000 m3 ultra-thin-film balloon made of a 3.4 μm thick polyethylene film. The balloon reached an altitude of 53.0 km (174,000 ft) to establish the first new world record in 30 years.
Airships can operate and persist, but only up to about 25 km. Generally, these airships are unmanned aerial vehicles (UAV) designed to operate as remotely operated aircraft (ROA). A team sponsored by the Army Space and Missile Defense Command and led by the Southwest Research Institute demonstrated a stratospheric airship (HiSentinel) in powered flight at an altitude of 74,000 feet (22.6 km) on Dec. 4, 2005. The United States Department of Defense Missile Defense Agency currently has plans to build a high-altitude airship as a platform for radar to enhance ballistic missile defense. The high-altitude airship is a lighter-than-air UAV designed to operate as a surveillance platform, telecommunications relay, or a weather observer. The airship would remain in the air for up to one month at a time in a quasi-geostationary position at a height of above 18 km (60,000 feet) using solar cells to provide electrical power. It will be about 150 m (500 feet) long and about 46 m (150 feet) in diameter, and operable to survey land in a 600-mile (970 km) diameter.
Satellites generally operate above 200 km. Several orbits are used for earth communication and observation. Some of the more commonly used orbits are low earth orbits, semi-synchronous (e.g., Molniya) orbits, and geosynchronous orbits. Given the rapid orbital decay of orbits below approximately 200 km, a low Earth orbit (LEO) is generally defined as an orbit between 200 km-2,000 km above the Earth's surface. LEO satellites generally move at about 11 km/s. A semi-synchronous orbit has an orbital period of about 12 hours. Relative to the Earth's surface, it has twice this period, and hence appears to go around the Earth once every day. Examples include the Molniya orbit and the orbits of Global Positioning System satellites. A satellite in Molniya orbit varies in altitude from about 1,500 km to about 40,000 km. A geosynchronous orbit is an orbit around the Earth matching the Earth's sidereal rotation. For an observer at a fixed location on Earth, a satellite in a geosynchronous orbit remains at approximately the same place in the sky during each day. A satellite in geostationary orbit is at an altitude of about 36,000 km. The very high cost and very high altitude limit the useful applications at geostationary orbit.
Most of the above approaches do not provide sufficient time and persistence over a location, or operate effectively at high non-orbital altitudes. Therefore, there is a need for a relatively stationary platform capable of operating effectively at high altitudes.